Data collected after underground coal-gasification field tests indicated that the concentrations of organic contaminants in the ground water decrease with time, apparently due to two natural processes - adsorption and biological degradation. Batch isotherm tests of the adsorption mechanism showed that 1) low-molecular-weight phenolic materials (the most prevalent contaminants) are the least likely to be adsorbed from the ground water, while high-molecular-weight polynuclear aromatics (PNA) are adsorbed most easily, 2) the adsorptive affinity of a compound increases with alkylation and ring addition, 3) the adsorptive affinity of aromatic hydrocarbons decreases progressively with insertion of oxygen or sulfur in the ring, insertion of nitrogen in the ring, amination of the ring, and conversion to the phenolic derivative, 4) the adsorption characteristics for a Hanna coal and a Texas lignite are similar, and 5) filtration cannot be used in laboratory studies of PNA because of adsorption to the filter. Analyses of the microbial composition of ground water from a UCG test site in Texas suggested that in situ biodegradation of UCG contaminants can occur. Microbial degradation depends on the concentration of dissolved oxygen; for maximal growth of organisms of UCG products, a limited concentration of oxygen - similar to levels measured in the ground water - is preferable to totally aerobic or anaerobic conditions. These microaerobic conditions might take part in the nitrification and nitrate respiration-dependent processes during biodegradation.